Duobinary is a well-known modulation format in fiber optic communications. Optical duobinary signals typically have two to three times the chromatic dispersion tolerance of conventional binary signals as well as a high tolerance to nonlinear penalties. The increased chromatic dispersion tolerance of duobinary transmission can provide significant cost savings in optical transmission systems by removing the need for dispersion compensation modules (DCMs).
One technique for realizing transmission of optical duobinary signals is to generate a three-level modulator drive signal by filtering a differentially encoded binary NRZ data stream with an electrical low-pass filter. The filtered drive signal drives a modulator to generate the duobinary signals for transmission across a transmission link to a receiver. This technique is typically referred to as the electrical low-pass filtered (LPF) duobinary approach. An example of an optical transmission system for transmitting electrical LPF duobinary signals is shown in FIG. 1. The differential encoder shown in FIG. 1 is used to pre-code the data before the low-pass filtering so that the generated optical duobinary signal (after digitization at the receiver) is the same as the original data.
Duobinary transmission systems are potentially cost effective commercial fiber optic transport solutions, particularly for metropolitan applications. However, in transmission links with low chromatic dispersion (e.g. chromatic dispersion compensated links or links spanning relatively short distances) or with back-to-back transmission, duobinary signals can suffer a 2-4 dB optical signal-to-noise (OSNR) penalty as compared to conventional binary signals.